南海海山玄武岩形成条件研究

A STUDY ON THE FORMING CONDITIONS OF BASALTS IN SEAMOUNTS OF THE SOUTH CHINA SEA

南海海山火山岩主要由碱性玄武岩,拉斑玄武岩、粗安质浮岩、英安岩等组成。火山岩中的矿物包裹体主要为非晶质熔融包裹体,反映出海山火山岩水下喷发冷凝较快的特点,并在碱性玄武岩及其地幔岩包体中发现流体熔融包裹体,表明该区上地幔部分熔融过程中曾经历过熔浆与流体不混熔的过程。四个海山火山岩直接测定的成岩温度分别为:中南海山碱性玄武岩1155～1185℃,宪北海山碱性玄武岩960～1200℃,拉斑玄武岩1040～1230℃,玳瑁海山拉斑玄武岩1245～1280℃,尖峰海山粗安质浮岩880～1140℃。中南海山及宪北海山碱性玄武岩的平衡压力分别为13.57×10~8Pa和8.8×10~8Pa。宪北海山地幔岩包体的辉石平衡温度为1073～1121℃,压力为(15.58～22.47)×10~8Pa,表明碱性玄武岩浆来源较深,已达到该区上地幔软流圈范围。

Volcanic rocks in seamounts of the South China Sea consist of alkali basalt, tholeiitic basalt, trachyandesitic pumice, dacite, etc. Inclusions in minerals of the volcanic rocks are mainly amorphous melt inclusions. which reflects the volcanic rocks are characterized by submarine (?)ption and rapid cooling on the seafloor. Furthermore, fluid-melt inclusions were discovered for the first time in alkali basalt and mantle-derived xenoliths, indicating a process of differentiation between magma and fluid in the course of mantle partial melting. Alkali basalt and inclusions may be formed in this nonhomogeneous system. Rock-forming temperatures of four seamounts were estimated as follows: the Zhongnan seamount alkali basalt 1155～1185℃; the Xianbei seamount alkali basalt 960～1200℃; tholeiitic basalt 1040～1230℃; the Daimao seamount tholeiitic basalt 1245～1280℃ and the Jianfeng seamount trachyandestic pumice 880～1140℃. Equilibrium pressures of alkali basalt in the Zhongnan and Xianbei seamounts are 13.57 and 8.8 kb respectively. Pyroxene equilibrium temperatures of mantle xenoliths from the Xianbei seamount were estimated at 1073～1121℃, and pressures at (15.58～22.47)×10~8Pa, suggesting a deepsource (e.g.the asthenosphere) for the alkali basalt.